Interpretive Summary: Problem –
Interaction with weed roots is thought to play a role in the ability of weed-suppressive rice varieties to control weeds. However, evaluation of these interactions under field conditions has been very challenging, largely because crop and weed roots become easily intertwined in the soil and cannot be separated without damage. We aimed to develop and field-test an accurate and efficient method that determines the relative amounts and distribution of rice and barnyardgrass roots in soils of flooded rice fields by measuring ratios of stable carbon isotopes present in root samples obtained from these fields.
Accomplishment –
The carbon isotope ratios were very consistent over the four years of the experiment, and the (negative) values for pure rice roots were more than double those for pure barnyardgrass roots. This facilitated the development of accurate standard concentration curves for measuring root mixtures of unknown rice and barnyardgrass content. About 50% more barnyardgrass root material was found in the top 2 inches of soil in plots of son-suppressive Lemont rice than in weed-suppressive PI 312777 rice.
Contribution –
This work represents the first successful use of a stable carbon isotope technique to determine subsurface root distributions of a weed-suppressive rice cultivar and barnyardgrass in flood-irrigated rice fields. It opens the way to more thorough exploration of the important interactions that may occur between the roots of weed-suppressive rice and troublesome grass weeds.

Technical Abstract:
Evaluation of crop-weed root interactions in soils under field conditions has traditionally been challenging, largely because roots of the crop and weed can become physically intertwined during plant development. Root interactions between weed suppressive rice and barnyardgrass under field conditions are not well understood. This paper introduces a stable isotope method that can quantify the amounts of roots of rice and barnyardgrass intermixed in flooded field soils. It relies on the biological principle that rice, a C3 (photosynthetic pathway) species, discriminates more effectively than barnyardgrass, a C4 species, against a relatively rare isotopic form (13C) of CO2. This results in different 13C:12C isotope ratios (expressed as ‘delta’ 13C) in root tissues of the two species. ‘Delta’ 13C values for monoculture barnyardgrass and rice grown in a standard flood-irrigated system were highly stable over four crop years, averaging approximately -13.12 ±0.80 and -28.5±0.11 ‰ (parts per 1000), respectively, based on analysis by an isotope ratio mass spectrometer. Standard concentration curves relating measured ‘delta’ 13C values to set proportions of rice:barnyardgrass root biomass or root carbon mass were well described by linear regressions, typically with regression coefficient values of 0.96 or greater. Quantities of intermixed rice and barnyardgrass roots sampled 0- to 5-cm-deep from soil between rice rows were estimated by extrapolation from standard curves based on ‘delta’ 13C values. About 50% more barnyardgrass root tissue was detected in plots of Lemont long-grain rice than in weed-suppressive PI 312777 indica rice, demonstrating the feasibility of using this stable carbon isotope method in flooded rice systems.